4 Bar Stability


Is it better for the tower of a four bar lift to be wider or narrower? Which one is more stable and easier to lift more weight?


Both can work. Be sure to cross brace the towers to each other to prevent racking, and support any gears on both sides to prevent them from driving apart from each other.


Thank you for answering. How would you cross brace them? Also, our lift is strange in the sense that it does not go up straight, instead moving in a small arc if you understand. It is a double four bar. Got any input on why this is happening and how to fix?
Drawing Lift.png


check how the weight is balanced out


make sure both stages of the lift are of equal length otherwise the arcs wont cancel each other out.


Oh ok, i’ll look at that.


What 2 stages?


There’s the first, outer stage. The bars on it need to be the same length as the second, inner stage.


the first 4 bar that is directly mounted to the base is the first stage. the second 4 bar that is attached to the end of the first 4 bar is the second stage. they must be the same length or you wont have perfect vertical motion.


I see two things in the diagram. Is that diagram in the third post correct? I’ll assume so for the moment. So…

  1. The first thing is that there is only a single four-bar drawn, and everyone seems to be talking about a DR4B. You even said “double four bar.” The upper bar (drawn diagonally lower-left to upper-right) could well swing with the rest of it as it done with a DR4B, though. Or should there actually be two lines like that and have them connected at the lower-left end.
  2. The upper bar (drawn diagonally lower-left to upper-right) extends beyond the base towers. That means it’s longer than the lower bars (drawn diagonally upper-left to lower-right). That’s what others are talking about. It should only reach to the base tower, not beyond it as shown in the drawing.


Yes the diagram is correct.For the first point, I just realized that I drew it wrong and there is actually two parallel bars. They are connected at the lower left end. The upper bar is longer and extends out past the base, yes. The reason we did that is so we could attach ourr flipping device without it getting stuck between the two bars. That’s part of the reason i asked if it would be better to make the base tower wider. If we made it wide enough, the flip arm could fit in between.


In other words, the diagram is not correct. The lengths are, but there are some missing pieces. That makes more sense of things.

I understand doing that, but that is exactly what is causing the issue you’re talking about. The radius of one circular path is different than the radius of the other, so the horizontal shifts don’t cancel out and the motion isn’t purely vertical.

Yes, you could make it wider to fit what you need between the towers. You could also just leave it as is and accept the little bit of an arc. But you have other options. You could stop between the towers as they are and use something like a 5-wide c-channels or similar (maybe with one edge cut off to make an L shape to squeeze in tighter) to connect the upper bars (drawn top-right to bottom-left) at their end (bottom-left). This way it still extends outward, but it’s the vertically pieces that extend outward instead of the swinging arm pieces. If my description isn’t clear, I can try to get a diagram posted soon.

Edit: Look at @Xenon 's ASCII art below to see what I’m saying, though I can still draw the c-channel if that bit didn’t make sense.


well then thats your problem. to extend the second stage, don’t lengthen your 4 bar, instead add some sort of extension to the end of the lift. take a look at my marvelous ASCII art:


Thanks for the drawing! I understand now. I think we’ll do that instead.


Yes your correct about the drawing haha sorry. I think I understand what your saying though. Makes sense and would be easier than widening it since everything is already made and cut to that length.


As i am working on it, I realized something else. The second set of bars doesn’t start at the same point as the first two do. Is that a problem? I attached a picture, sorry about the quality.
Also, the distance between the bars for each set is different, does that matter?


As long as it’s below 18", it’ll be fine.


Ok, it is


The relative height of the tower and the vertical section in the middle (right-hand side of the diagram) is irrelevant. It’s more important to base that on getting the gears in properly and getting the height right for your purposes.


We finally got the lift to work nicely. But, as soon as we added an attachment to the front, more problems occurred. The lift started to go back on itself and the flip arm got stuck in the robot… It struggles to go up as soon as we added more weight.